Electron gun structure



Oct. 6, 1959 w. R. BEAM ETAL 2,907,916

ELECTRON GUN STRUCTURE Filed Aug. 17, 195e INVENToRs WALTER R. BEAM v Y BY RUNALD E. KNEEHILI 4r Niv United StatesV Patent Of 2,907,916v- ELEcrRoN GUN STRUCTURE Walter R: Beam, Hamilton Square, 'and ll-{onild C.

Knechtll, Cranbury, N.J.,'assignors to Radio Corporanon of America, a corporation of Delaware This invention relates to electron. gunslffor cathode ray tubes or kinescopes, and itfhas-forits object: to provide a novel high voltage limiting aperture. electron gun of improved performance.

Another object of the invention is to decreaseuthe spotsize inan electron gun of the above type for a given current and gun length.

A further object is to provide an electron gun of th above type in which less` voltage is required at the limiting aperture for a given peak current, and in which less power is dissipated by this aperture.

Still another object is'to provide a high voltage limiting aperture electron gun having improved cut-off and beam modulation characteristics. y Y

Various other objects and advantages will lbe apparent as the nature'of the invention is more fully disclosed.

In any situation where a very 'small spot on the phosphorscreenof a cathode ray tube or kinescope is required, the normal type of electron gun isrnot suitable. The spot obtained therefrom is of variable size, depending on the beam current. Furthermore', thespot edges are not sharply delined.V In a high voltage limiting aperture gun, thespot is practically of constant size because the object is nota crossover of electron paths, but a mechanically defined aperture. With the Vlimiting aperture operated at high voltage (500 rv. or

more) the eiects of space charge and of chromatic aberrations become negligible, and the sharpness of definition of the spot is determined by the aberration of lenses alone.

The present invention provides a high Voltage limiting aperture gun of improved performance which includes a cathode and electrode means for producing a convergent electron beam, a modulating electrode having an aperture which is disposed in the path of the convergent beam, and an object-forming electrode disposed along the path of travel of the beam beyond said modulating electrode. The said object-forming electrode has a limiting aperture formed therein for the passage of the beam, the size of which aperture is smaller than the transverse size of the beam for sharply defining the transverse size and configuration of the beam upon its emergence from said limiting aperture. The novel construction, hereinafter described in detail, results in a signiiicant reduction in the magnification of the spot with respect to the limiting aperture, and accordingly permits the production of a smaller size spot with more sharply defined edges than in the case of previous electron guns of this general type.

'I'he invention is described more in detail in connection with the accompanying single sheet drawing in which Fig. l is a sectional View of a cathode ray tube employing an electron gun in accordance with this invention, Fig. 2 is an enlargedV sectional view of the electron gun of Fig. 1.

, The electron gun shown in the drawing is mounted within a tube envelope'26 and comprises a cathode K with a concave electron emitting surface 1 at one end.v

A 2,907,916 Patented Oct. 6, 1952,49

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2 Disposed beyond the cathode K is an annular electrode G1 mounted by supports 2 coaxially with the cathode. A second electrode G3 is positioned by supports 3 beyond the annular electrode G1 and has a central aperture 4 formed therein through which the electron beam passes. The structure thus far described comprising the cathode K and the electrodes G1 and G3 is of the type generally known as a Piercew'gun (see Spangenbergs Vacuum Tubes, pp. 449-465).

The Pierce gun K-'-G1G3 produces a convergence electron beam which lpasses through straightening and object-forming electrodes G3 and G4, respectively. The straightening velectrode which. vis designated generally as G3, is mounted by isupports 5 and is provided with a central aperture 6 through -which the beam passes, the aperture 6 being coaxially aligned with the elements K- G1+G3 of the Pierce gun. The aperture 6 is formed in a thin flat disc 7 which is secured to a d ishedA annular member9 so that the aperture 6 is offset toward the cathode.

The object-forming electrode G4, which is mounted by supports 10, is disposed along the direction of travel of the beam beyond ,the straightening electrode G3. Electrode G4 is ofgshouldered cylindrical conguration and comprises a angedcylindrical body portion 11 having an open end upon which a thincentrally apertured guide disc 12 is mounted. Secured to the guide disc '12 on the sidetoward the cathode K is a thicker disc '13, preferably formed of molybdenum. The disc 13 has a central limiting aperture 14 formed therein coaxially aligned with cathode K and the Yapertures in electrodes G1, G2 and G3.` In certainlembodiments of the invention` the limiting. aperture 14 may be rec. tangular, say/,0.015 inch long by 0.004 inch wide, where a spot of rectangular Vshape is desired on the screen of the tube in which the gun is used.

Terminals 16, 17,1418, 19 and 20 are provided for the individual application of suitable potentials to the cathode K and electrodes G1, G2, G3 and G4, respectively.

The operation of the electron gun is as follows:

To obtain a spot of given size, from electrons emerging from a limiting aperture of given size, such as aperture 14, the magnification M of the gun is xed. The amount of magnification M is, of course, desirably made as small as possible, to keep the spot on the screen as small as possible. The magnification M is given by the following expression:

M Vaperture sin (aperture) V sin (0 Dimension of spot Dimension of aperture Screen SCleeIl) screen 2 )sul 0screen 'I a =Jeath m X Screen Tcathode Where Icrth Vis the current density at the cathode. In a limiting-aperture gun, there is also a thermal limitation at the aperture:

J 111600 Vaperture i 2 0 J max aperturecath .y Teath Sm aperture Taking the ratio of these maximum current densities, we obtain: f

2 Vscreen Sm Imax screen screen 1 Jmar aperture Vaperture M2 This indicates that if the limitation is reached at the aperture, the same will be true at the screen, hence the problem of designing such a gun is resolved into obtaining the most current at the least value of Vp sin2 03D. Applying theseconsiderations, in one embodiment of the invention, we yoperate the straightening electrode G3 with a biasing potential of 150 volts and with a potential of 1500 volts Vapplied to ,object-forming electrode G4, the inter-aperture spacings `4 6, and 6414 -each being about 1/16 inch. This produces a value of 2.1 for Wmme sin @aperture `witha beam current of 1 milliampere as contrasted with .other known forms of gun' 2 sm 0aperture surface into a convergent electron beam, a convergence construction which mayl give arvalue of 6 or more for this important perfomance factor.

The electron beam from cathode K is convergent until it has passed through the aperture L4 in G2.' Then the amount of convergence is decreased by the action of the strong aperture lens caused by G2 and the `decelerating eld in the region from G2 to G3. After passing through the aperture 6 in G3, the beam is accelerated in a uniform field which has a straightening out effect, the reduction vin convergence angle being about 1/3 by the time the beam reaches theV limiting aperture 14.

The beam passing through the limiting aperture 14 is then focused on screen `21 by focusing electrodes 22, 23 and wall coating 24 which are maintained at appropriate voltages during tube operation. The beam is deected over screen 21 by magnetic deflecting fields provided by coils mounted infyoke 25. Y

What is claimed is:

1. An electron gun structure comprising a cathode means having a concave electron emitting surface, an annular electrode adjacent to said cathode surface and having a frusto-conical surface facing away from said cathode for focusing electrons emitted from said cathode vreducing straightening electrode having an aperture formed therein which is disposed in the path of said beam for passage of said beam therethrough, and an object-forming electrode disposed along the path of travel of said beam beyond said straightening electrode, said object-forming electrode having a limiting aperture formed therein for passage of said beam therethrough, the size of said limiting aperture being smaller than the transverse size of said beam for sharply dening the transversejsize and configuration of said beam upon its emergence from said limiting aperture.

2. 'The Acombination according to claim 1, in which said aperture in said straightening electrode is larger than said limiting aperture.

3. The combination according to claim 2, further comprising means for maintaining said object-forming electrode .at a .high potential with respect to said cathode means, andl meansfor maintaining said straightening electrode at a potential considerably less thanfthe potential on said object-forming electrode, whereby said beam is accelerated during its passage from said straightening electrode to said object-forming electrode and Ythereby made more parallel to the axis of `the gun.

4. The ncombination according to claim 3, ,in which the magnitude of said potential on said .object-forming electrode is at least ve times as great as said potential on said modulating electrode.

References Cited in the file of this patent UNITED STATES PATENTS 

